The Castellan was a high-ranking Time Lord who oversaw the Chancellery Guard. The Guard, in turn, had responsibility for security in the Capitol and particularly the security of the President. This position made the Commander of the Chancellery Guard his or her subordinate. It also put the Castellan in direct conflict with the Coordinator of the Celestial Intervention Agency, who saw to external threats against Gallifrey itself or the integrity of the timeline in general. (AUDIO: Pandora)

Castellans often served on the High Council, but not always; Castellan Spandrell, for example, was not a High Council member. (TV: The Deadly Assassin, Arc of Infinity, The Five Doctors) In Spandrell's time, the Castellan's duties involved him more frequently with the "plebeian classes" of Gallifrey than with suspect Time Lords. (TV: The Deadly Assassin) Castellan Kelner, by contrast, was second in line to the Presidency, after the Chancellor. (TV: The Invasion of Time)

The position was mandated by some of the most ancient and powerful Gallifreyan laws. (AUDIO: Lies) Even so, the laws of Gallifrey apparently gave the Presidency wide discretion in deciding which matters were handled by the Castellan, and which by the Coordinator. Lady President Romana, for instance, once simply chose which of the two she wanted to handle a matter. (AUDIO: Pandora)

There may have been a Castellan for each of the Chapter Houses, with one of them serving as the 'duty Castellan' in overall charge of the rest. Often the Castellans had served in the Chancellery Guard prior to their elevation. Both Maxil and Andred served in this way. Others appear to have been political appointments, like Kelner (TV: The Invasion of Time) and Wynter. (AUDIO: Lies, Pandora, PROSE: Lungbarrow)

Holders of the office of Castellan

• Marnal (PROSE: The Gallifrey Chronicles)
• Spandrell (TV: The Deadly Assassin)
• Kelner (TV: The Invasion of Time)
• Rannex (PROSE: Divided Loyalties)
• Un-named male Castellan (killed) (TV: Arc of Infinity, The Five Doctors)
• Lode (PROSE: Love and War)
• Voran, a member of the Dromeian Chapter (The Infinity Doctors universe (PROSE: The Infinity Doctors)
• Andred (PROSE: Lungbarrow), until his disappearance
• Wynter (AUDIO: Lies)
• Leela (AUDIO: Insurgency), acting Castellan
• Andred (AUDIO: Insurgency), a second time, until his death
• Maxil (AUDIO: Warfare)
• Annos (AUDIO: Mindbomb), acting Castellan